This invention relates generally to the preparation of wet process phosphoric acid, and more particularly to an improvement on a conventional wet process for the preparation of a phosphoric acid solution with a special view to enhancing the content of uranium in the phosphoric acid solution.
Phosphate rocks of natural occurrence generally contain about 100-200 ppm of uranium. In the wet process manufacture of phosphoric acid through wet decomposition of phosphate rock with a mixed acid consisting of sulfuric acid and recycled phosphoric acid, most of uranium contained in the phosphate rock transfers into a slurry of calcium sulfate in phosphoric acid solution. Since wet process phosphoric acid is manufactured in an enormous quantity, recovery of uranium from wet process phosphoric acid solution has long been tried although the uranium content in the solution is not so high. At present solvent extraction methods are predominant in this art, and in some firms the recovery of uranium from wet process phosphoric acid has already been put into industrial practice.
According to the form of calcium sulfate as an important by-product, practical techniques for the manufacture of wet process phosphoric acid can be classified into five groups; namely, anhydrite process, hemihydrate process, dihydrate process, dihydrate-hemihydrate process, and hemihydrate-dihydrate process. Among these processes, the most popular is the dihydrate process wherein calcium sulfate is separated in the form of dihydrate, i.e. gypsum in a narrow sense. The hemihydrate process wherein calcium sulfate is precipitated as hemihydrate, the hemihydrate-dihydrate process wherein calcium sulfate is intermediately formed as hemihydrate and subsequently converted to dihydrate by treatment in a mixed acid and the dihydrate-hemihydrate process wherein calcium sulfate is formed initially as dihydrate and subsequently converted to hemihydrate by treatment in a mixed acid at a temperature above a dihydrate-hemihydrate transition point (after separation of the dihydrate from the phosphoric acid solution as the mother liquor or alternatively without separating from the solution) have been developed mainly in expectation of the growth of calcium sulfate crystals that are low in the content of phosphoric acid and easy to separate from the acid solution and the acquirement of a phosphoric acid solution which is as high as possible in the P.sub.2 O.sub. 5 content. The water of crystallization of calcium sulfate that precipitates in a wet process for preparation of phosphoric acid is determined by the concentration of P.sub.2 O.sub.5 in the liquid phase of the reaction system and the reaction temperature. In this regard the phase equilibrium relationships are represented by the famous Nordengren's phase diagram.
Japanese Patent Applications Publication Nos. 43(1968)-17415, 45(1970)-14405 and 45(1970)-27323 show the particulars of the aforementioned dihydrate-hemihydrate process. We have recognized that when this process is performed according to the teachings of these patent documents uranium originated in phosphate rock is captured almost exclusively by the hemihydrate gypsum. For example, we have obtained the following experimental result. Phosphate rock was decomposed according to the teachings of Japanese Patent Application Publication No. 45(1970)-27323 by using a mixed acid of sulfuric acid and phosphoric acid so as to give a slurry of dihydrate gypsum. The supernatant phosphoric acid solution of this slurry (hereinafter this solution will be referred to as crude phosphoric acid) had a uranium content of 80 ppm, whereas the uranium content in the dihydrate gypsum was only 4 ppm. Sulfuric acid was added to this gypsum slurry so as to adjust the concentration of excess sulfuric acid in the slurry to the level of 3-10%, and the slurry was maintained at a temperature above the dihydrate-hemihydrate transition point in the presence of seed crystals of hemihydrate gypsum until complete conversion of the dihydrate gypsum to hemihydrate gypsum. By filtration the thus treated slurry was divided into hemihydrate gypsum and phosphoric acid for recycling. It was confirmed that the hemihydrate gypsum obtained through this procedure had a uranium content of 48 ppm, while the uranium content in the phosphoric acid had reduced to 11 ppm.
Regarding the preparation of phosphoric acid by a wet process in which hemihydrate gypsum is formed, it has become clear that about a half of uranium originally contained in the phosphate rock and transferred into the acid solution is captured by the hemihydrate gypsum.
Also it is known that tetravalant ions of uranium are more readily captured by dihydrate gypsum than the hexavalent ions of uranium. When dihydrate gypsum is dispersed in a phosphoric acid solution containing uranium in the form of tetravalent ions, the uranium content in the gypsum reaches about four times as high as that in the case of the acid solution containing uranium in the form of hexavalent ions. ("Recovery of Uranium from Phosphoric Acid (for phosphatic fertilizer)", published by Power Reactor and Nuclear Fuel Development Corporation of Japan, pp. 4-5, 1975.)